The housing of a pump strainer is usually under negative pressure (i.e., suction), thereby subjecting the strainer cover to the same type of force. The cover is usually under such negative pressure with two exceptions. First, if the pump is installed below water level and not running, a relatively low positive pressure is exerted on the cover. Most covers will withstand such relatively low positive pressure. Second, when the entire pool plumbing system, including the pump, is pressure tested at or during initial installation or at a later time for service or repair purposes, a relatively high positive pressure is exerted on the cover. Most strainer covers cannot withstand the relatively high pressure exerted during such testing, unless specially designed to do so.
In the past, pressure-testable pumps have employed two basic types of covers. One type employs a cover which is adapted to be threadedly attached to the strainer housing. This type of cover complicates normal operation (i.e., when pressure testing is not required), because the threaded attachment between the cover and the strainer housing makes it difficult to remove the cover, which in connection with the normal operation of the pump would be removed about once a week, daily, or even several times a day depending upon the application.
The other type of pressure testable pump employs a cover which is clamped onto the strainer housing by screw-type hand knobs. However, in order to make this type of pump pressure testable, the cover must be made from iron or a suitable metal, which is not corrosion resistant and/or is very expensive.